1. Field of the Invention
The subject invention relates to mobile cellular networks, and more particularly, to a method for allocating channels to mobile subscribers traveling through a handover region extending across the boundary of adjacent cells of a cellular network.
2. Description of the Related Art
A mobile cellular network is a radio system which provides radio communication for mobile subscribers over a large geographical area. Cellular networks generally consist of a plurality of base stations arranged in a systematic pattern throughout a geographical area to define a plurality of adjoining cells of between 2 and 20 kilometers in radius.
Typically, a mobile subscriber equipped with a mobile radio unit in a particular cell communicates through a radio channel provided by the base station of that cell. When a mobile subscriber is traveling between adjacent cells of a network, a handover must be performed whereby the mobile subscriber terminates communication with the base station of one cell and initiates communication with the base station of another cell. A handover may be blocked however, if a free channel is not available within the target cell, resulting in the disruption of service to the mobile subscriber. The probability of handover failure, typically described by the forced termination of calls, is a major factor in evaluating the performance of a cellular system.
Techniques for minimizing or reducing handover disruptions have been described in the prior art and include both non-reservation based and reservation based handover systems. In non-reservation based systems, handover requests are prioritized based upon a queuing scheme, such as, for example, a first-in-first-out scheme. In such a system, a new call originating within the target cell will not be granted a channel until all of the handover requests in the queue are served.
In reservation based systems, a fixed or dynamically adjustable number of free channels may be reserved in each cell of a network for handover requests. For example, U.S. Pat. No. 5,081,671 to Raith et al. describes a system for reducing handover interruptions by allocating a predetermined number of reserve channels in each cell of a network for the handover requests of mobile subscribers. However, in such a system, free channels reserved for handovers may go unutilized due to a lower than expected number of handover requests. Thus, the total carried traffic will be reduced even when free channels are in existence.
Another method for reducing handover disruption is described in U.S. Pat. No. 5,189,734 to Bailey et al. which includes the steps of forming a subset of cells adjacent the cell within which a mobile subscriber is traveling and requesting the base stations in each of these cells to reserve a channel in the event of a handover to one of the base stations. In such a system, free channels reserved for handover requests in a plurality of cells will go unutilized for a period of time while the mobile subscriber is traveling within the current cell, thereby reducing the overall efficiency of the cellular network.
Clearly, a need exists for an improved reservation based system which reduces the probability of handover failure in a mobile cellular network without significantly increasing call blocking probability and decreasing overall carried traffic.